International patent application WO 94/11226 discloses a brake force booster of this general type. The prior art brake force booster suffers from the disadvantage of the effect of annular surfaces on the valve member of the control valve which are delimited by the second and third sealing seats, on the one hand, and the boundary of the mentioned pneumatic chamber, on the other hand, especially in the event of independent activation by switching on the electromagnets. This condition affects adversely the use of the independently actuatable brake force booster especially in control operations. One of the annular surfaces to which a pneumatic differential pressure is applied during the independent activation of the brake force booster produces a force component which counteracts the independent actuating force generated by the electromagnet and tends to close the control valve. This necessitates compensating the force component by an increase of the independent actuating force which must be generated by the electromagnet.
Therefore, an object of the present invention is to disclose measures which permit eliminating the disturbing effects of the mentioned force component.
According to the present invention, this object is achieved because a permanent pneumatic connection between the pneumatic chamber and the vacuum chamber is provided, and in that both the second and the third sealing seat and the valve member in its part delimiting the pneumatic chamber in the control housing have almost identical diameters which, preferably, range in a tolerance width of 1 mm. It is preferred that the pneumatic chamber is delimited by a cylindrical part which cooperates with a sealing lip designed on the valve member.
In a preferred aspect of the subject matter of the present invention, the valve member has two sealing surfaces which preferably have an annular configuration and are arranged one behind the other in the actuating direction, and the first and the third respectively the second sealing seat are movable into abutment with the sealing surfaces.
The present invention will be explained in detail in the following description of an embodiment, making reference to the accompanying drawings.